Optical phase shifter

ABSTRACT

A optical phase shifter is provided for adjusting an optical phase of light propagating therethrough along an optical axis. The optical phase shifter includes first and second transparent slides defining a cavity therebetween. A sheet is received in the cavity and has first and second sides. The sheet includes a rigid inner portion alignable with the optical axis and is moveable along the optical axis between a first position and a second position. A tuning structure is operatively engageable with the rigid inner portion of the sheet to selectively move the rigid inner portion of the sheet along the optical axis so as to adjust the optical phase of light propagating through the optical phase shifter.

REFERENCE TO GOVERNMENT GRANT

This invention was made with government support under OD008678 awardedby the National Institutes of Health. The government has certain rightsin the invention.

FIELD OF THE INVENTION

This invention relates generally to the adjustment of the optical pathlength of light passing through a media, and in particular, to a tunableoptical phase shifter with a transmissive and wide aperture medium.

BACKGROUND AND SUMMARY OF THE INVENTION

When light propagates through a media, the optical path length of thelight depends on the effective index of refraction of the media. As isknown, the optical phase may be adjusted when light propagates through amedia having a desired optical path length. Optical phase tuning ishighly desired in various precision and imaging tools used forapplications in industry and scientific research. Among various opticalphase shifters (OPS), electro-optic modulators are frequently used,though mainly in communication systems.

Electro-optics are comprised of components, devices (e.g., lasers,light-emitting diodes (LEDs), waveguides, etc.) and systems whichoperate by the propagation and interaction of light with varioustailored materials. Other phase shift methods employ fiber-optic basedmodulators, which suffer light loss, liquid crystal phase shifters,which can be slow, or mirror based systems, which require complex,precise mechanical controls.

With the advancement of imaging systems, such as optical coherencetomography, these imaging systems require optical path length modulationin one arm of an interferometer setup to produce noninvasivecross-sectional imaging in biological systems. In phase shiftinginterferometry (PSI), an OPS with a wide aperture and a linear medium isrequired. Other optical imaging systems, such as phase contrastmicroscopy, rely on a static OPS. Existing optical phase shift solutionseach come with certain drawbacks. A tunable optical phase shifter with atransmissive and wide aperture medium is desirable for thesimplification and miniaturization of optical systems.

Therefore, it is a primary object and feature of the present inventionto provide a tunable optical phase shifter with a transmissive and wideaperture medium.

It is a further object and feature of the present invention to providean optical phase shifter that simply and easily adjusts the opticalphase of light propagating through a media having a desired optical pathlength.

It is a still further object and feature of the present invention toprovide an optical phase shifter that is compatible with current imagingtools and is inexpensive to manufacture.

In accordance with the present invention, a tunable optical phaseshifter is provided for adjusting an optical phase of light propagatingtherethrough along an optical axis. The tunable optical phase shifterincludes a sheet having first and second sides and including a rigidouter portion interconnect to a rigid inner portion alignable with theoptical axis by a complaint ring. The rigid inner portion is moveablealong the optical axis between a first position and a second position. Afluid is provided on the second side of the sheet and is engageable withthe rigid inner portion of the sheet for exerting a pressure thereon. Apressure generator is operable to selectively vary the pressure of thefluid against the rigid inner portion of the sheet to move the rigidinner portion of the sheet along the optical axis so as to adjust theoptical phase of light propagates through the optical phase shifter.

The rigid inner portion of the sheet may be fabricated from a negativephotoresist and the fluid may be non-conductive. The pressure generatoralso includes a conductive fluid extending about at least a portion ofthe non-conductive fluid. The pressure generator also includes aplurality of interdigitated electrodes positioned in spaced relation tothe second side of the sheet. The plurality of interdigitated electrodesare operatively connectable to a voltage source. The voltage sourcesupplies an adjustable voltage such that the pressure of the fluidagainst the rigid inner portion of the sheet varies in response to amagnitude of the voltage supplied to the plurality of interdigitatedelectrodes by the voltage source. At least one spacer may be positionedbetween the plurality of interdigitated electrodes and the second sideof the sheet for spacing the sheet from the plurality of interdigitatedelectrodes.

In accordance with a further aspect of the present invention, an opticalphase shifter is provided for adjusting an optical phase of lightpropagating therethrough along an optical axis. The optical phaseshifter includes first and second transparent slides defining a cavitytherebetween. A sheet is received in the cavity and has first and secondsides. The sheet includes a rigid inner portion alignable with theoptical axis and is moveable along the optical axis between a firstposition and a second position. A tuning structure is operativelyengageable with the rigid inner portion of the sheet to selectively movethe rigid inner portion of the sheet along the optical axis so as toadjust the optical phase of light propagating through the optical phaseshifter.

The sheet includes a rigid outer portion interconnect to the rigid innerportion by a complaint ring. The compliant ring urges the rigid innerportion toward the first position. At least one spacer spaces the sheetfrom the second slide. The rigid inner portion of the sheet isfabricated from a negative photoresist. The tuning structure includes afirst fluid on the second side of the sheet that is engageable with therigid inner portion of the sheet for exerting a pressure thereon. Thefirst fluid is non-conductive. The tuning structure also includes asecond fluid extending about at least a portion of the first fluid. Thesecond fluid is conductive. A plurality of interdigitated electrodes arespaced from the second side of the sheet. The plurality ofinterdigitated electrodes are operatively connectable to a voltagesource. The voltage source supplies an adjustable voltage such that thepressure of the first fluid against the rigid inner portion of the sheetvaries in response to a magnitude of the voltage supplied to theplurality of interdigitated electrodes by the voltage source.

In accordance with a still further aspect of the present invention, anoptical phase shifter is provided for adjusting an optical phase oflight propagating therethrough along an optical axis. The optical phaseshifter includes first and second transparent slides defining a cavitytherebetween. A sheet is received in the cavity and has first and secondsides. The sheet includes a rigid inner portion alignable with theoptical axis and moveable along the optical axis between a firstposition and a second position. A tuning structure is operativelyengageable with the rigid inner portion of the sheet to selectively movethe rigid inner portion of the sheet along the optical axis so as toadjust the optical phase of light propagating through the optical phaseshifter. The tuning structure includes a first fluid on the second sideof the sheet and engageable with the rigid inner portion of the sheetfor exerting a pressure thereon. The first fluid is non-conductive. Asecond fluid extends about at least a portion of the first fluid. Thesecond fluid is conductive. At least one electrode is spaced from thesecond side of the sheet and communicates with the conductive fluid. Theelectrode is operatively connectable to a voltage source. The voltagesource supplies an adjustable voltage. The pressure of the first fluidagainst the rigid inner portion of the sheet varies in response to amagnitude of the voltage supplied to the electrode by the voltagesource.

The sheet includes a rigid outer portion interconnect to the rigid innerportion by a complaint ring. The compliant ring urges the rigid innerportion toward the first position. At least one spacer spaces the sheetfrom the second slide. The rigid inner portion of the sheet isfabricated from a negative photoresist and the at least one electrode isone of a plurality of interdigitated electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings furnished herewith illustrate a preferred construction ofthe present invention in which the above advantages and features areclearly disclosed as well as others which will be readily understoodfrom the following description of the illustrated embodiment.

In the drawings:

FIG. 1 is schematic, isometric view of a tunable optical phase shifterin accordance with the present invention;

FIG. 2 is a cross sectional view of the optical phase shifter of thepresent invention in an initial state taken along line 2-2 of FIG. 1;

FIG. 3 is a cross sectional view of the optical phase shifter of thepresent invention, similar to FIG. 2, showing the optical phase shifterin a displaced state;

FIG. 4 is a cross sectional view of the optical phase shifter of thepresent invention taken along line 4-4 of FIG. 2;

FIG. 5 is a schematic view of the optical phase shifter of the presentinvention interconnected to a voltage source; and

FIGS. 6A-6F are schematic views showing exemplary steps in thefabrication of the optical phase shifter of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 1-5, an optical phase shifter in accordance with thepresent invention is generally designated by the reference numeral 10.Optical phase shifter 10 include first and second spaced slides 12 and14, respectively, having inner surfaces 18 and 20, respectively,directed towards each other and defining cavity 16 therebetween. Firstand second slides 12 and 14, respectively, are fabricated from atransparent material, e.g. glass. However, first and second slides 12and 14, respectively, may be fabricated from other materials withoutdeviating from the scope of the present invention. The outer peripheriesof first and second slides 12 and 14, respectively, are bonded to eachother in any conventional manner so as to fluidically seal cavity 16from the environment outside of optical phase shifter 10, for reasonshereinafter described. Alternatively, a three dimensional printedchamber may be placed between first and second slides 12 and 14,respectively, and glued to thereto to provide a sealed cavity.

As best seen in FIGS. 2-3, sheet or layer 22 is positioned between innersurfaces 18 and 20 of first and second slides 12 and 14, respectively,in cavity 16. It is contemplated for layer 22 and first and secondslides 12 and 14, respectively, to lie in corresponding generallyparallel planes. By way of example, layer 22 fabricated from is a highcontrast, epoxy based photoresist such as SU-8. However, layer 22 may befabricated from other materials without deviating from the scope of thepresent invention. Layer 22 includes an upper surface 24 spaced from anddirected towards inner surface 18 of first slide 12 and a lower surface26 spaced from and directed towards inner surface 20 of second slide 14.

Layer 22 further includes a reduced thickness portion, generallydesignated by the reference number 30. In the depicted embodiment,reduced thickness portion 30 is generally ring-shaped having an inneredge 32 defining a generally circular, optical section 34 of layer 22and an outer edge 36. As hereinafter described, it is intended forreduced thickness portion 30 to allow optical section 34 of layer 22 tomove axially along an optical axis in response to a hydraulic pressurethereon. Alternatively, reduced thickness portion 30 may be fabricatedfrom a material with reduced young modulus compared to the rest of layer22 to facilitate the displacement of optical section 34, as hereinafterdescribed. Outer periphery 38 of layer 22 and outer edge 36 of reducedthickness portion 30 of layer 22 define a support portion 40 of layer 22therebetween. For reasons hereinafter described, it is contemplated foroptical section 34 of layer 22 and support portion 40 of layer 22 to berigid to resist the curvature thereof. A plurality of interdigitatedelectrodes 42 are patterned on inner surface 20 of second slide 14, FIG.4. Electrodes 42 are aligned with layer 22 and includes first and secondlines 44 and 46, respectively, projecting laterally therefrom alonginner surface 20 of second slide 14. Each line 44 and 46 extendsoutwardly of cavity 16 and terminates at a corresponding terminal 48 and50, respectively, FIG. 5, which are selectively connectable to voltagesource 78. Insulator 52 overlaps the outer periphery of plurality ofinterdigitated electrodes 42 and includes a lower surface 54 bonded toelectrode 42 and an upper surface 56 directed towards lower surface 26of layer 22. A plurality of spacers 58 extend between upper surface 56of insulator 52 and support portion 40 of layer 22 so as to maintainlayer 22 generally parallel to first and second slides 12 and 14,respectively.

Optical phase shifter 10 further includes first and second fluids orliquids 60 and 62, respectively, providing in cavity 16. First andsecond liquids 60 and 62, respectively are immiscible and may beconsidered incompressible. Furthermore, first and second liquids 60 and62, respectively, may have closely matching densities, in order toincrease shock, vibration and acceleration resistance of the opticalphase shifter 10 and have different refractive indices. First liquid 60is non-conductive and is provided in a lower portion 16 a of cavity 16between lower surface 26 of layer 22 and upper surface 56 of insulator52. Second liquid 62 is conductive and is provided between upper surface24 of layer 22 and inner surface 18 of first slide 12 and about outerperiphery 64 of first liquid 60. It is noted by providing rigid portionsof layer 22, namely, optical section 34 of layer 22 and support portion40 of layer 22 between two immiscible liquids, namely, first and secondliquids 60 and 62, respectively, curvature is prevented from forming atthe optical interfaces. For reasons hereinafter described, second liquid62 is drawn into lower portion 16 a of cavity 16 between lower surface26 of layer 22 and upper surface 56 of insulator 52 in response to theapplication of voltage to electrodes 42 via electrowetting.Alternatively, second liquid 62 may be a liquid with high dielectricconstant which is capable of providing a dielectrophoretic force onfirst liquid 60, instead of electrowetting.

In operation, optical phase shifter 10 is provided in an initial statewherein the plurality of interdigitated electrodes 42 are electricallyisolated from voltage source 78. Optical phase shifter and/or lightsource 70 are positioned such that light rays 72 generated by lightsource 70 are directed toward optical phase shifter 10 and aligned withoptical section 34 of layer 22. It is intended for light rays 72 totravel along an optical axis having a path normal to outer surface 74 offirst slide 12 so as to sequentially pass through first slide 12, secondliquid 62, optical section 34 of layer 22, first liquid 60 and secondslide 14.

In its initial state, FIG. 2, optical phase shifter 10 exhibits anoptical path length (OPL) along the optical axis of:OPL₁ =n _(A) d _(A) +n _(B) d _(B)  Equation (1)wherein: subscript “1” denotes the initial state of optical phaseshifter 10; n_(A) is a refractive index of second liquid 62, n_(B) isthe refractive index of first liquid 60, d_(A) is the thickness ofsecond liquid 62 through which light rays 72 travel; and d_(B) is thethickness of first liquid 60 through which light rays 72 travel.

Once optical phase shifter 10 is properly positioned, the plurality ofinterdigitated electrodes 42 of optical phase shifter 10 may beelectrically connected to variable voltage source 78 such that voltagesource 78 provides a user selected, variable voltage to the plurality ofinterdigitated electrodes 42. In response to application of the userselected voltage to the plurality of interdigitated electrodes 42,second liquid 62 is drawn into lower portion 16 a of cavity 16 betweenlower surface 26 of layer 22 via electrowetting or dielectrophoresis,FIG. 3. Since first and second liquids 60 and 62 may be consideredincompressible, the drawing of second liquid 62 into lower portion 16 acauses the pressure generated by first liquid 60 in lower portion 16 aof cavity 16 on optical section 34 of layer 22 to increase. Thisincreased hydraulic pressure of first liquid 60 in lower portion 16 a ofcavity 16 causes the displacement of optical section 34 of layer 22along the optical axis against the resilient retaining force of reducedthickness portion 30 of layer 22. With optical section 34 of layer 22displaced along the optical axis in response to the user selectedvoltage, optical phase shifter 10 may be designated as being in adisplaced state, FIG. 3.

It can be appreciated that the volume of first liquid 60 drawn to lowerportion 16 a of cavity 16 is dependent on the magnitude of the userselected voltage provided to the plurality of interdigitated electrodes42. Hence, by varying the magnitude of the user selected voltageprovided to the plurality of interdigitated electrodes 42, the hydraulicpressure exerted by first liquid 60 on optical section 34 of layer 22may be adjusted, and consequently, the magnitude of the displacement ofoptical section 34 of layer 22 along an axis coincident with the travelpath of light rays 72 may be controlled.

The displacement of optical section 34 of layer 22 along the opticalaxis induces an optical path difference (OPD), and thus, a phase shiftcompared to the initial state. More specifically, in the displacedstate, optical phase shifter 10 exhibits an optical path length (OPL)according to the expression:OPL₂ =n _(A) d _(A1) +n _(B) d _(B2)  Equation (2)wherein: subscript “2” denotes the displaced state of optical phaseshifter 10; n_(A) is the refractive index of second liquid 62; d_(B) isthe refractive index of first liquid 60; d_(A2) is the thickness ofsecond liquid 62 through which light rays 72 travel; and d_(B2) is thethickness of first liquid 60 through which light rays 72 travel.

As such, the optical path difference (OPD) may be calculated as thedifference in the OPL between the initial state and the displaced statein accordance with the expression:OPD_(1→2)=(n _(B) −n _(A))Δd  Equation (3)wherein: n_(A) is the refractive index of second liquid 62; n_(B) is therefractive index of first liquid 60; and Δd is the distance opticalsection 34 of layer 22 of optical phase shifter 10 has been displaced.

In view of the foregoing, it is understood that the optical phase shiftof light rays 72 traveling through optical phase shifter 10 may becalculated according to the expression:OPS=2π·OPD_(1→2)/λ  Equation (4)wherein: OPS is the optical phase shift of light rays 72 thought opticalphase shifter 10; OPD_(1→2) is the difference in the OPL of light rays72 with optical phase shifter 10 in the initial state and with theoptical phase shifter 10 in the displaced state; and λ is the wavelengthof light rays 72 traveling through optical phase shifter 10.

In view of the foregoing, it can be appreciated that by selectivelyvarying the displacement of optical section 34 of layer 22 along theoptical axis by varying the magnitude of the voltage supplied to theplurality of interdigitated electrodes 42, a user may accuratelytune/adjust the optical phase of the light rays 72 passing throughoptical phase shifter 10. While the thickness of optical section 34 oflayer 22 of optical phase shifter 10 adds an extra OPL to optical phaseshifter 10, it is noted that the thickness of optical section 34 oflayer 22 remains constant during device operation. Hence, the extra OPLdoes not alter the calculation of the OPS. Similarly, first and secondslides 12 and 14, respectively, add an extra OPL to optical phaseshifter 10. However, it is noted that the thicknesses of first andsecond slides 12 and 14, respectively, remain constant during deviceoperation, and hence, do not alter the calculation of the OPS.

Referring to FIGS. 6A-6F, an exemplary process for fabricating theoptical phase shifter 10 of the present invention is depicted. It can beappreciated that optical phase shifter 10 may be fabricated in othermanners without deviating from the scope of the present invention.Referring to FIG. 6A, a 200 nanometer (nm) Tantalum layer is sputteredand patterned on inner surface 20 of second slide 14, e.g, a four (4)inch bottom glass wafer, to form the plurality of interdigitatedelectrodes 42. Insulator 52, e.g. a two (2) micrometer (μm) layer ofSU8, is spin coated and patterned on the plurality of interdigitatedelectrodes 42, as shown in FIG. 6B. Thereafter, a two hundred (200) μmSU8 layer is patterned on upper surface 56 of insulator 52 to constructthe plurality of spacers 58, FIG. 6c . The partially constructed opticalphase shifter is then exposed to an oxygen plasma treatment to increasethe surface energy of the SU8 to facilitate the subsequent bonding tothe SU8 components, heretofore described.

A wafer 80 having a sacrificial copper layer with a thickness of sevenhundred fifty (750) nm deposited thereon is provided, FIG. 6d . Thesacrificial copper layer is patterned to provide alignment marks 82 forsuccessive steps hereinafter described. A two (2) μm SU8 layer ispatterned on the copper layer, FIG. 6e . It is intended for the two (2)μm SU8 layer 84 to form reduced thickness portion 30 of layer 22.Thereafter, a first fifty (50) μm SU8 layer 86 is patterned on the two(2) μm SU8 layer to form optical section 34 of layer 22 and supportportion 40 of layer 22, FIG. 6f . Finally, a second fifty (50) μm SU8layer is patterned on the first fifty (50) μm SU8 layer to provide four(4) pairs of semicircular retention and alignment dimples thereon.

Wafer 80 is roughly aligned with second slide 14 and wafer 80 and secondslide 14 are pressed together. The four (4) pairs of semicircularretention and alignment dimples patterned on first fifty (50) μm SU8layer engage corresponding retention apertures provided in second slide14 such the wafer and second slide 14 latch together. Thereafter, aselected pressure, e.g. 15 standard atmosphere units (atm), is appliedto wafer 80 and second slide 14 and the latched wafer and second slide14 combination is placed in a vacuum furnace. The latched wafer andsecond slide 14 combination is heated to a desired temperature, e.g.130° C., for a selected period of time, e.g. 3 hours, to bond thelatched wafer and second slide 14 combination together. Thereafter, thebonded wafer and second slide 14 combination cooled to room temperatureand cut to size with a dicing saw. The bonded wafer and second slide 14combination is deposited in a copper etchant solution for a selectedtime period so as to cause the sacrificial copper layer to release wafer80. First and second lines 44 and 46, respectively, are electricallycoupled to the plurality of interdigitated electrodes 42, for example,by a conductive silver epoxy glue. Second slide 14 is interconnected tofirst slide 12 in any conventional manner, e.g. with a marine gradeepoxy glue, to fluidically seal cavity 16 in optical phase shifter 10.

Alternatively, as heretofore described, a three dimensional printedchamber may be positioned between first and second slides 12 and 14,respectively, and glued thereto to provide cavity 16. Second liquid 62,e.g., a silicone oil, may injected through an opening (not shown) intoportion 16 a of cavity 16 within optical phase shifter 10. The openingis then sealed with a fast curing epoxy. Subsequently, the rest ofcavity 16 in optical phase shifter 10 is filled with first liquid 60,e.g. water.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter, which is regarded as theinvention.

We claim:
 1. A tunable optical phase shifter for adjusting an opticalphase of light propagating therethrough along an optical axis,comprising: a sheet having first and second sides and including: a rigidouter portion having an inner edge; a rigid inner portion having anouter edge and being moveable with respect to the rigid outer portionalong the optical axis between a first position and a second position;and a compliant ring interconnecting the outer edge of the rigid innerportion to the inner edge of the rigid outer portion; a fluid on thesecond side of the sheet and engageable with the rigid inner portion ofthe sheet for exerting a pressure thereon; and a pressure generatoroperable to selectively vary the pressure of the fluid against the rigidinner portion of the sheet to move the rigid inner portion of the sheetalong the optical axis so as to adjust the optical phase of lightpropagates through the optical phase shifter.
 2. The tunable opticalphase shifter of claim 1 wherein the rigid inner portion of the sheet isfabricated from a negative photoresist.
 3. The tunable optical phaseshifter of claim 1 wherein the fluid is a non-conductive fluid.
 4. Thetunable optical phase shifter claim 3 wherein the pressure generatorincludes a conductive fluid extending about at least a portion of thenon-conductive fluid.
 5. The tunable optical phase shifter of claim 1wherein the pressure generator includes a plurality of interdigitatedelectrodes positioned in spaced relation to the second side of thesheet, the plurality of interdigitated electrodes operativelyconnectable to a voltage source.
 6. The tunable optical phase shifter ofclaim 5 wherein the voltage source supplies an adjustable voltage andwherein the pressure of the fluid against the rigid inner portion of thesheet varies in response to a magnitude of the voltage supplied to the aplurality of interdigitated electrodes by the voltage source.
 7. Thetunable optical phase shifter of claim 5 further comprising at least onespacer between the plurality of interdigitated electrodes and the secondside of the sheet for spacing the sheet from the plurality ofinterdigitated electrodes.
 8. The tunable optical phase shifter of claim1 wherein: the rigid outer portion lies in first plane; the optical axisis generally perpendicular to the first plane; and the rigid innerportion is generally planer and parallel to first plane in the firstposition.